The present invention relates to a television receiver which utilizes a liquid crystal matrix display panel, and in particular to such a television receiver whereby a larger number of elements can be driven with a satisfactory level of display contrast than has been possible in the prior art, without the need for utilizing large-capacity memory circuit means.
Liquid crystal display panels are used in various electronic devices, such as calculators, timepieces, etc, where only a relatively small amount of data need be displayed, and in which they provide the advantages of extremely thin shape together with a very low degree of power consumption. In order to provide a relatively large size of display, comparable to that attainable by utilizing a cathode ray tube display, a matrix arrangement of the liquid crystal display elements is employed, i.e. with a set of horizontally aligned and a set of vertically aligned drive electrode lines disposed on opposite faces of the display panel, defining display elements at the intersections of these electrode lines. However, a limit to the number of display elements of such a liquid crystal matrix display panel is set by the fact that the display contrast drops to an unacceptable level, if the number of display elements increases above a certain value. More specifically, in the case of a liquid crystal matrix display panel in which lines of display elements, e.g. horizontally aligned rows of display elements, are successively addressed by scanning signals, satisfactory display contrast together with a sufficiently wide viewing angle cannot be obtained if the number of rows of display elements is increased above approximately 50 to 60. This limitation is a result of cross-talk effects occurring between the electrodes, and is set by the physical characteristics of the liquid crystal material together with the drive signal waveforms which must be employed.
In the case of a liquid crystal matrix display panel television display having a picture size comparable to that of a conventional CRT display, it is necessary to provide approximately 500.times.700 picture elements, to obtain sufficient display resolution. With a very small size of display screen, used in a miniature television receiver, the number of display elements can be reduced. However even in this case, it is necessary to provide an array of at least 120.times.160 picture elements. Thus, using a simple conventional line-by-line scanning system for a liquid crystal matrix display panel, it has not been possible to provide a display having a sufficiently large number of display elements for use in a television receiver. Various ways of overcoming this problem have been proposed, to enable the number of rows of picture elements which can be driven to be increased. In one such method, as described in detail hereinafter, multiplex drive is utilized. Specifically, each horizontally aligned timing electrode of the matrix, to which periodically generated timing signals are applied to scan the rows of picture elements, is arranged to drive two or more rows of picture elements by time-division operation. This enables the number of rows of picture elements which can be driven with a satisfactory level of display contrast to be substantially increased, i.e. the level of display contrast can be made approximately equal to that obtainable with a simple sequential scanning drive method in which one-half the number of rows of display elements are used. For example, it is possible to provide such a liquid crystal matrix display panel having a total of 60 horizontally aligned timing electrodes, each of which drives two lines of picture elements, with 320 vertically aligned segment electrodes. Such a multiplex display panel will provide the same number of display elements as a simple, i.e. non-multiplexing type of matrix display panel, having 160 segment electrodes and 120 timing electrodes. However with the multiplex display panel, a total of (60+320)=380 connecting leads are required coupled between the drive electrodes and the peripheral circuits which generate the drive signals. In the case of the simple, non-multiplex drive method driving the same number of display elements, only (120+160)+280 connecting leads are required between the drive electrodes and the peripheral circuits. Thus a substantially greater number of connecting leads between the display panel and the drive circuits are required, and in addition a greater number of output terminals must of course be provided on the integrated circuits forming these drive circuits.
In addition, such a multiplexing method has the disadvantage that it becomes necessary to introduce connecting lead portions between the vertically aligned drive electrodes, leading to greater difficulty in arranging the electrode pattern configuration, and so to significantly increased manufacturing cost. In addition, a substantial amount of display area is occupied by these connecting lead portions, reducing the aperture ratio of the display panel and so acting to reduce the display contrast and so to some extent defeating the intended objective.
The multiplex drive method has the further disadvantage that cross-talk can occur between the connecting lead portions disposed between the drive electrodes on the display panel, thereby tending to further reduce display contrast.
For these reasons, it is not practical to implement multiplexing by a factor of more than two, i.e. with each timing electrode driving two rows of picture elements, so that it is not possible to provide a sufficient number of display elements for television display purposes using the multiplexing drive method alone.
With another method which has been proposed in the prior art, the display is divided into two regions, i.e. an upper and a lower region, which are driven separately. Incoming video signal data is stored in a large-capacity memory circuit, i.e. having sufficient capacity to store image data for the entire display screen. After being stored, image data for the top half of the display and that for the lower half is read out from the memory circuit simultaneously, while scanning signals are applied to select the rows of display elements of the upper and lower display regions in synchronism. In this way, each half of the display is scanned twice in succession during each vertical scanning interval of the television broadcast signal, so that the effective level of display contrast attained is equivalent to that of a liquid crystal matrix display panel utilizing a simple sequential scanning drive method but having one-half the number of rows of picture elements.
This prior art method has the great disadvantage that the memory circuit required must have a very large storage capacity and in addition must operate at very high write and read speeds, so that the cost of this memory circuit will substantially increase the overall manufacturing cost of the television receiver.
There is therefore a requirement for drive means to be employed in a television receiver having a liquid crystal matrix display panel whereby a sufficiently large number of rows of display elements can be provided for acceptable picture resolution together with a sufficiently high degree of display contrast, and whereby the disadvantages of the prior art drive methods described above can be eliminated, so that such a television receiver can be manufactured at lower cost than has been possible hitherto.